.TH ARP 8 "5 Jan 1999" "net-tools" "Linux Programmer's Manual"
.SH NAME
arp \- manipulate the system ARP cache
.SH SYNOPSIS
.B arp 
.RB [ \-vn ] 
.RB [ "\-H type" ] 
.RB [ "-i if" ] 
.B -a 
.RB [ hostname ]
.PP
.B arp 
.RB [ \-v ]
.RB [ "\-i if" ] 
.B "\-d hostname"
.RB [ pub ]
.PP
.B arp 
.RB [ \-v ] 
.RB [ "\-H type" ] 
.RB [ "\-i if" ] 
.B -s hostname hw_addr
.RB [ temp ] 
.PP
.B arp 
.RB [ \-v ] 
.RB [ "\-H type" ] 
.RB [ "\-i if" ] 
.B -s hostname hw_addr
.RB [ "netmask nm" ] 
.B pub
.PP
.B arp 
.RB [ \-v ] 
.RB [ "\-H type" ] 
.RB [ "\-i if" ] 
.B -Ds hostname ifa
.RB [ "netmask nm" ] 
.B pub
.PP
.B arp 
.RB [ \-vnD ]
.RB [ "\-H type" ] 
.RB [ "-i if" ]
.B -f [filename]

.SH DESCRIPTION
.B Arp
manipulates the kernel's ARP cache in various ways.  The primary options
are clearing an address mapping entry and manually setting up one.  For
debugging purposes, the
.B arp
program also allows a complete dump of the ARP cache.
.SH OPTIONS
.TP
.B "\-v, \-\-verbose"
Tell the user what is going on by being verbose.
.TP
.B "\-n, \-\-numeric"
shows numerical addresses instead of trying to determine symbolic host, port
or user names.
.TP
.B "\-H type, \-\-hw-type type"
When setting or reading the ARP cache, this optional parameter tells
.B arp
which class of entries it should check for.  The default value of
this parameter is
.B ether
(i.e. hardware code 0x01 for IEEE 802.3 10Mbps Ethernet).
Other values might include network technologies such as
.RB "ARCnet (" arcnet ")"
,
.RB "PROnet (" pronet ")"
,
.RB "AX.25 (" ax25 ")"
and
.RB "NET/ROM (" netrom ")."
.TP
.B "\-a [hostname], \-\-display [hostname]"
Shows the entries of the specified hosts.  If the
.B hostname
parameter is not used,
.B all
entries will be displayed.
.TP
.B "\-d hostname, \-\-delete hostname"
Remove any entry for the specified host.  This can be used if the
indicated host is brought down, for example.
.TP
.B "\-D, \-\-use-device"
Use the interface
.BR ifa "'s"
hardware address.
.TP
.B "\-i If, \-\-device If"
Select an interface. When dumping the ARP cache only entries matching
the specified interface will be printed. When setting a permanent or
.B temp
ARP entry this interface will be associated with the entry; if this
option is not used, the kernel will guess based on the routing
table. For
.B pub
entries the specified interface is the interface on which ARP requests will
be answered. 
.br
.B NOTE:
This has to be different from the interface to which the IP
datagrams will be routed.
.TP
.B "\-s hostname hw_addr, \-\-set hostname"
Manually create an ARP address mapping entry for host
.B hostname
with hardware address set to
.B hw_addr
.  The format of the hardware address is dependent on the hardware
class, but for most classes one can assume that the usual presentation
can be used.  For the Ethernet class, this is 6 bytes in hexadecimal,
separated by colons. When adding proxy arp entries (that is those with
the 
.BR pub lish 
flag set a 
.B netmask 
may be specified to proxy arp for entire subnets. This is not good
practice, but is supported by older kernels because it can be
useful. If the
.B temp
flag is not supplied entries will be permanent stored into the ARP
cache.
.br
.B NOTE:
As of kernel 2.2.0 it is no longer possible to set an ARP entry for an 
entire subnet. Linux instead does automagic proxy arp when a route
exists and it is forwarding. See 
.BR arp (7)
for details.
.TP
.B "\-f filename, \-\-file filename"
Similar to the
.B \-s
option, only this time the address info is taken from file
.B filename
.  This can be used if ARP entries for a lot of hosts have to be
set up.  The name of the data file is very often
.IR /etc/ethers , 
but this is not official. If no filename is specified /etc/ethers
is used as default.
.sp 1
The format of the file is simple; it
only contains ASCII text lines with a hostname, and a hardware
address separated by whitespace. Additionally the 
.BR "pub" , " temp" " and" " netmask"
flags can be used.
.LP
In all places where a
.B hostname
is expected, one can also enter an
.B "IP address"
in dotted-decimal notation.
.P
As a special case for compatibility the order of the hostname and 
the hardware address can be exchanged.
.LP 
Each complete entry in the ARP cache will be marked with the
.B C
flag. Permanent entries are marked with
.B M
and published entries have the
.B P
flag.
.SH FILES
.I /proc/net/arp,
.br
.I /etc/networks
.br
.I /etc/hosts
.br
.I /etc/ethers
.SH SEE ALSO
rarp(8), route(8), ifconfig(8), netstat(8)
.SH AUTHORS
Fred N. van Kempen, <waltje@uwalt.nl.mugnet.org> with a lot of improvements
from net-tools Maintainer Bernd Eckenfels <net-tools@lina.inka.de>.
